Heat bluing a blade?

[skullworks]

I saw some that have the cool purple blue color and wondered how you did it with out screwing up the blade temper.

Looks like this.

 

[Lin Rhea]

I would think the blueing heat has to be no higher than the drawback temps of the blade steel. Lin

[Ragnarok Forge]

I recently read a post where a 20 minute etch in vinegar followed by 2 hours at 320 degrees gave a very nice blue. I plan to try it on a couple of blades I am working on.

[David Ricke]

I have used some cold blues with varying degrees of success. I have also tryed to heat blue but the temp is too high for a blade. I have used it on damscus bolsters.  Knifemaker